Proximity sensor on Galaxy S4 (Air Gestures)

There is a good youtube video from a Google I/O session called "the sensitive side of Android" that explains how you could do something similar. Basically you use the light sensors and then you register a drop. This is quite simple but you gotta remember that changing light sources while running the app can affect the results of this feature.

That said I think they use hand detection on the front camera in order to pull off all those gestures and that would be quite complex. Also I haven't seen any api that makes it simple to do hand detection.

Yon can find it what you want

The Samsung Mobile SDK 1.5 beta1 consists of 16 independent packages; including 'Pass' and 'Motion', which allow you to make use of the Finger Print and Activity Recognition features of the Samsung GALAXY S5.

New: Accessory, Accessory File Transfer, Companion UI Profile, Health, Pass, Remote Sensor

Update: Chord, Gesture, Motion(Activity Recognition)

You can import one or more packages into your application as your project requires. For more information on the contents of each package, please check below.

http://developer.samsung.com/samsung-mobile-sdk#gesture